The present invention relates to a portable torque measuring device for measuring torque, especially a torque which is involved in the assembling of bearings or other machine elements.
For example, for inspection of the assembling torque associated with the assembled state of bearings for the differential gear mechanism of an automobile, the wattmeter type torquemeter for measuring the power loss of a driving electric motor has heretofore been used. This is because there has been a demand for a small, light-weight torque measuring device in order to facilitate the use of such devices in the production shop.
The wattmeter type torquemeter comprises an electric motor and speed-reducer for rotating the shaft of an object to be measured, and a wattmeter for measuring the consumed power in said electric motor.
Let W be the consumed power in the motor, E the motor voltage and I the motor current. Then, EQU W=E.I
on the other hand, between the load torque T and the motor voltage and current E and I, there is the following relation: EQU T=A(I)I1/NE(I)-
where N is the rpm and Tm is the mechanical loss torque. Here, A(I) and E(I) are functions of said current I, and as a method of compensating this, means, such as the one disclosed in Japanese Patent Opening No. 136468/1976, have been proposed. However, such prior art means require a special compensation-purpose electronic device, and it is technically and economically difficult to impart the capability of computing these compensation functions A(I) and E(I) to the portable torquemeter used in the field. Therefore, the wattmeter type torquemeter heretofore used is uncalibrated and poor in accuracy of measurement and there are cases where it cannot be put to practical use owing to voltage variations, etc.
As for the electrical torque-measuring method, the strain gauge, load cell, etc. have heretofore been used, but there are variations in their zero point caused in principle, making it necessary to make sufficient temperature compensation in order to assure accuracy which is stabilized for a long period of time over a wide range of measurement. Further, a torquemeter using a high precision strain guage, load cell or the like is complicated in construction and so large that it cannot be used as a portable torquemeter.
Further, in the measurement of a stick-slip type varying torque which is found among assembling torques in specially surface-treated bearings (and which has the characteristic of locally reducing the damping characteristic of the bearing as the rpm of the bearing increases, and is produced with a relatively low cycle), the power loss-detecting type torquemeter heretofore used is poor in vibration damping characteristc with respect to low-frequency varying torque, with the meter pointer fluctuating, so that reading is sometimes impossible or errors of reading are liable to occur. This phenomenon would not lead to an error of measurement if a time-consuming averaging operation is carried out, still it forms a major error factor where the meter is used for short-time inspection in the field.